The present invention relates to integrated circuits, and more particularly to multi-chip modules (MCMs).
The following documents relate to integrated circuits and may be of background interest:                U.S. pre-grant patent publication 2009/0267238 (Oct. 29, 2009; Joseph et al.).        U.S. pre-grant patent publication 2012/0020027 (Jan. 26, 2012, Dungan et al.)        
A multi-chip module (MCM) is an assembly of multiple components, with one or more components being integrated circuits (ICs), such that the assembly can be used like a single semiconductor integrated circuit. A usual (not multi-chip) semiconductor integrated circuit can be manufactured as a wafer or as a die (single-chip IC) formed in a wafer and later separated from the wafer (when the wafer is diced); multiple ICs can be manufactured in a wafer simultaneously. The ICs and possibly discrete circuits and possibly other components (like non-semiconductor packaging substrates including printed circuit boards, interposers, and possibly others) can be assembled in an MCM. In this disclosure, the words “die” and “chip” are synonymous.
FIG. 1 illustrates an MCM which includes multiple dies 110 (110F.1, 110F.2, etc.) attached to a packaging substrate 120, e.g. a wiring board (WB) such as a PCB or an interposer. WB 120 has interconnect lines (not shown) for interconnecting the dies. The MCM may combine dies of different types, and some of these types are illustrated in FIG. 1. In this example, dies 110F include a CPU (central processing unit) 110F.1; an IVR die (Interactive Voice Recognition) 110F.2; an audio chip 110F.3 which may include a microphone and/or a speaker and/or audio signal processing circuitry; a Power Management Integrated Circuit (PMIC) 110F.4; an actuator die 110F.5; an RF (radio frequency) communication die 110F.6; a GPU (graphics processing unit) 110F.7; an optics die 110F.8 (e.g. optic transducer and/or processing circuitry), a Solid State Drive (SSD) 110F.9; Random Access Memory (RAM) 110F.10; Digital Signal Processor (DSP) 110F.11; and a sensor chip 110F.12 (e.g. optical sensor, pressure sensor, or some other type). We will refer to the CPU, IVR and other dies shown in the drawing as “function dies” or “function chips”.
To reduce manufacturing costs, the WB can be made using organic (e.g. insulating polymer) and/or ceramic and/or glass and/or composite materials. Such WBs can be inexpensively fabricated using molding, printing, or other techniques. For example, a WB can be a laminate of ceramic or organic or composite material layers with conductive lines on each layer which together form an interconnect network that interconnects the WB's contact pads (not shown) attached to the dies. Such WBs can be less expensive to make than those made of silicon. However, the minimum feature size of organic or ceramic or composite WBs is typically larger than for silicon chips. In particular, the minimal interconnect width and the spacing between the interconnects can be 1000 times larger than in silicon. This is partially due to the fact that many organic, ceramic, and composite materials are not as flat as polished silicon, i.e. they have a rougher surface; therefore, photolithography is less precise. Further, such WBs are often patterned using coarser and less expensive methods than photolithography, such as screen printing or laser ablation. Also, the conductive and other features may have to be thicker than for silicon chips. We will call such WBs “coarse WBs” for ease of reference. The term “WB” includes both coarse and non-coarse (e.g. semiconductor or glass) WBs unless noted otherwise.
Thus, the coarse WB circuitry is larger, and has larger pitches between conductive lines and solder balls and other features. A silicon or glass WB provides denser packed circuitry (with smaller pitch), but is more expensive.
In view of the disadvantages of coarse WBs, a coarse WB can be supplemented by a silicon interposer positioned between at least some of the chips and the coarse WB. Thus, some of the chips are attached to the silicon interposer rather than WB. The silicon interposer has contact pads on top for attachment to the chips, and has other contact pads on the bottom for attachment to the WB. The interposer has through-substrate vias (TSVs) used to connect its top contact pads to the bottom contact pads. However, the TSVs are expensive to fabricate, and they are especially difficult to fabricate if the interposer is thick. But thin interposers are hard to handle, they easily break, and their warpage complicates the manufacturing and creates stresses that can break the MCM during operation.